Heat exchanger



HEAT EXCHANGER 2 Sheets5heet 1 Original Filed Nov. 16, 1961 IN VEN TORS.OLIVER E CAMPBELL NORMAN E4 PENNELS A TIORNEYS.

1967 o. F. CAMPBELL ETAL 3,

HEAT EXCHANGER Original Filed Nov. 16. 1961 2 Sheets-Sheet 2 INVENTORS.OLIVER F. CAMPBELL NORMAN E. PENNELS A TTORNE V5.

United States Patent C) 3,303,828 HEAT EXCHANGER Oliver F. Campbell,Chandler, Ariz., and Norman E. Pennels, Olympia Fields, 111., assignorsto Sinclair Research, Inc., Wilmington, Del., a corporation of DelawareOrigi 2] application Nov. 16, 1961, Ser. No. 152,733, now Patent No.3,256,924, dated June 21, 1966. Divided and this application Sept. 24,1965, Ser. No. 505,216

2 Claims. (Cl. 122--352) This application is a division of applicationSerial No. 152,733, filed November 16, 1961, now Patent No. 3,256,924.

This invention relates to a novel relatively small package apparatus forrecovering heating values from waste flue gases, particularly from fluegases produced by the regenerator of a hydrocarbon catalytic crackingoperat1on.

In petroleum catalytic conversion processes the catalyst employed iscontinuously regenerated by burning off carbonaceous matter thatdeposits on the catalyst. Since catalysts are deactivated by sinteringat high temperatures, the regenerator temperatures are controlled andlimited to usually a maximum temperature of about 11001200 F. Lowtemperatures coupled with limited quantities of air fed to theregenerator result in incomplete combustion of the coke and producesso-called regenerator gas or regenerator flue gas, which containscatalyst fines and, in addition to inert materials such as nitrogen,water vapor and carbon dioxide, small amounts of hydrocarbons and somecarbon monoxide, e.g. about 1 to 15% by volume with about 47% beingaverage.

The sensible heat of the flue gases and the latent heat represented bythe carbon monoxide and small amounts of hydrogen and hydrocarbons whichburn to carbon dioxide and water constitute a potentially valuablesource of heating values. The heating values are commonly recovered byuse of what is known in the art as CO boilers. Conventional CO boilerscontain a combustion furnace and a steam generator which is usually afluid heat exchanger comprising a lower water drum and a larger steamdrum placed vertically above the water drum and connected by a pluralityor bank of tubes and pro viding communication between the interiors ofthe drums and tubes. Water is supplied to the steam drum and flows downsome of the tubes and perhaps special downcomer tubes. Steam is producedby allowing the combustion gases resulting from the burning ofregenerator flue gases in the furnace section to pass, generally acrossthe tubes between the drums and on the outside of the tubes in indirectheat exchange relationship with the water in the plurality of tubes.

Unlike normal steam generators, i.e. where ordinary fuel (i.e. high Btu.fuel) is burned and Where the combustion value of the combustion gasesis on the order of about 1000 to 2000 or more Btu/cu. ft., the heatvalue of the regenerator flue gas (i.e. low B.t.u. fuel) is on the orderof only about 15 to 35 B.t.u./cu. ft. To attain steam production fromregenerator flue gases, which have a heat value about 100 times lessthan that found in the ordinary fuel burned in ordinary steam boilers,and the resulting combustion gases extremely large amounts of gases mustpass through the heat exchanger. To handle these flue gases inquantities economically justifying a flue gas recovery operation, thefluid heat exchanger units are necessarily of high capacity andnecessarily of a very large size and surface area. The actual size ofthe CO 'boiler to utilize the regeneration gases produced varies withthe size of the fluid cracking units. There are many large fluidcracking units in existence which produce volumes of regeneration gasesfrom which can be obtained on the order of 200,000 to 450,000 lbs. ofsteam per hour. The

Patented F eb. 14, 1967 field construction of CO boilers of a size largeenough to yield this quantity of steam is generally not only justifiedby the amount of steam produced but by the fact that it is actuallyrelatively cheaper to field construct a large CO boiler than one ofsmaller size. However, there are many smaller fluid cracking units whosesteam-producing poten tial is only about 50,000 to 100,000 lbs/hr. butto field construct CO boilers for them is in most cases too expensive.There remains, therefore, a need for a CO boiler to accommodate thesesmaller fluid cracking units.

Although a definite need exists for a CO boiler small enough to betransported for assembly at the desired site, yet of suflicient capacityto produce the desired volumes of steam, prefabricated, portable COboilers, unlike ordinary steam boilers, are unheard of. Sound reasonsexist for factory production of such boilers rather than construction atthe sites. It should be noted, for example, that conventional steamgenerators are welded together. This requires inspection, such as X-rayinspection, of the separate and multitudin-ous welds as well as, in manycases, preheating of the segments to be welded, and heat treatment ofthe welds. Such operations can be carried on most eflectively as shopoperations where the weld inspection apparatus and the heat treatmentequipment may be most effectively maintained and operated. Moreover, theoperations are conducted more cheaply in shops, i.e. indoors, whereinweather interruptions are eliminated and special tools, andbetter-trained personnel are available. However, a CO boiler of ordinarydesign, or even a heat exchanger alone built using conventionalprinciples and having a large enough capacity to generate the requiredamount of steam for low heat value gases, is too big to be transportedfrom the factory to the site on currently available carriers.Consequently what CO boilers exist have been built at the sites of theiruse and where such expensive construction is not possible, due to lackof the technical personnel needed, or due to cost factors, theregenerator off gases are wasted. It has been proposed that a heatexchanger of smaller size yet capable of incorporation in a CO boiler toproduce the desired quantities of steamrnight be obtained by increasingthe number of tubes connecting the drums of the fluid heat exchanger.This solution, however, presents a problem, for increasing the number oftubes in a conventional flow path for the gases increases the velocityof the combustion gases passmg in heat exchange relationship with thetubes. Since regenerator flue gases invariably contain catalyst fines,increasing the velocity of the gases causes undue erosion and damage tothe tubes by the catalyst fines. Furthermore, there is a limitation asto how many tubes may be rolled into each boiler drum since a certainarea in ligament must be left in the drums for maximum strength andsteam production.

The present invention provides apparatus for recovering heat values fromlarge volumes of waste flue gases, rnost advantageously from catalyticregenerator flue gases which apparatus is made up of several components,of such design that the assembled boiler is capable of absorbing thenecessary heat, that is, having a maximum capacity of up to about100,000 lb./hr. of steam at 600 p.s.i.g. and 750 F. and yet eachcomponent may be made small enough in size to be shop prefabricated andthe major components, boiler and furnace, shipped to any part of theUnited States for connection together at the site. For example, currentsize limitations in every part of the United States for shipping an itemby railway are ordinarily: maximum width, 10 feet 8 inches and maximumheight, 12 feet 3 inches. However, since furnaces are more easily madethan the heat exchanger and readily supplied locally, the furnace, incases where cross-country transportation is unnecessary or not desired,can be of larger size.

The apparatus of the present invention includes a specifically designedcombustion chamber or furnace and boiler. The flow path for combustiongases in the heat exchanger is generally normal to the heat exchangertubes rather than parallel. Such a flow path enables two or moreprefabricated heat exchanger units to be connected to provide a serialpath for the combustion gas through the number of prefabricated heatexchange units desired for full utilization of the heat values of thecombustion gas.

The dimensions of the heat exchanger are reduced to transportableproportions by providing a plurality of steam and water drums and byproviding heat exchange tubes generally parallel to each otherthroughout the hotgas contacting passage. Also, the steam and waterdrums are generally excluded from the insulated chamber through whichthe burner gases flow, since their capacity for heat exchange by heatgain from hot gases or heat loss to the atmosphere is comparatively low.The heat exchanger is generally lower in height and the contactingpassage is longer than in heat exchangers known in the art. Theprovision of a plurality of steam drums at the top and water or muddrums at the bottom of the heat exchanger accomplishes several desirablepurposes. First, it reduces the height of the heat exchanger whileproviding for the same capacity of steam or water. Secondly, itmaintains the strong cylindrical shape of the drums, rather than givethem a flattened and therefore weaker design. Thirdly it provides a moreextensive total surface for the drums, so that more heat exchanger tubesmay be provided without decreasing the amount of ligament below what isrequired for the inetgrity of each drum. Overall, this feature of thepresent invention makes best use of the available room, i.e. the sizelimitations for shipping (10'8" width by 125 height, maximum). Itprovides maximum flow area for gases while at the same time keeping thevelocity down.

The furnace, as mentioned above, is a separate component from the heatexchange boiler and also may be made in a size small enough for railtransportation anywhere in the United States. It is designed to handlethe extremely large amounts of off-gas produced by the ordinary catalystregenerator and to bring these gases in a confined space to the ignitiontemperature of the carbon monoxide and to burn the carbon monoxide witha minimum amount of an oxygen-containing gas, such as air, andsupplemental fuel to avoid a substantial increase in the already largeamount of gas which is handled by the heat exchanger. An increase in thealready large volume of gas increases the problem of keeping thevelocity down in the boiler. These functions are performed by employinga refractory-lined furnace and by arranging for spin entry of gases tothe furnace chamber for the creation of an area of thorough turbulenceto mix the heated off-gas with air and the products of combustion of theauxiliary fuel burner. Louvers and angular slots are used to spin thegases at their entry to the furnace to provide a zone of primary mixingand a very simple, rugged device is provided to create furtherturbulence or a secondary mixing zone, near enough to the area ofoff-gas heating to insure complete heating to ignition temperature andmixing to bring carbon monoxide into contact with oxygen, yet far enoughfrom the flame source of heat to prevent unstable flame conditionscharacterized by pulsation or blowing out. The turbulence is caused by asimple ring in the flow-path of the gases which is made of a refractorymaterial and which can be manufactured or repaired by ordinary masonrytechniques. The importance of employing a refractory-lined furnaceresides in the fact that it also lowers the quantity of air and fuelrequired since it prevents loss of heat to the boiler tubes until the COgas is burned. Employment of furnaces characterized by high heat loss,as for instance, waterwall furnaces, are unsuitable in that more fuel isneeded in these furnaces to obtain the necessary combustion temperature.The use of more fuel, as aforementioned,

means an increase in the already large volume of gas which in turnincreases the problem of keeping the velocity down.

A preferred form of the apparatus is illustrated in the accompanyingdrawings. It is to be understood that the apparatus as illustrated maybe modified in certain particulars as will be evident to those skilledin the art. The invention will be described with reference to theaccompanying drawings in which:

FIGURE 1 is an advantageous compact arrangement of the apparatus inoperation;

FIGURE 2 is a horizontal cross-section, in part, of the furnace of thepresent invention along line 2-2 of FIG- URE 1, and

FIGURE 3 is a verticalcross-section of the boiler 'of the presentinvention along the line 3-3 of FIGURE 1.

Referring to the drawings, the apparatus includes a furnace 1, desirablycylindrical and preferably having a circular cross-section; The furnaceis lined with a suitable refractory material 3 and can be of a size thatpermits transportation. A preferred refractory material ishightemperature insulating brick. This brick is made of alumina-silicamixed with sawdust so that on burning during its preparation, thesawdust decomposes to leave air pockets throughout the brick. The airpockets formed contribute to the production of a brick of low heatconductivity and at the same time a refractory material of relativelylight weight, a feature highly desirable in that it decreases theshipping and handling weight of the prefabricated CO boiler of thepresent invention.

At one end of the furnace there is provided burner means 6, preferablycentrally located within a conduit 9 containing an oxygen-containing gasinlet 12 and fuel line 15 which conducts fuel from a source (not shown)to the burner means where it is burned to provide the flame and heatnecessary for recovering the heat values of carbon monoxide. The conduit9 of the burner means extends across an annular chamber 18 provided witha plurality of louvers or registers 21 circumferentially disposed in theannular chamber about conduit 9 of the burner means. An annular conduit24 surrounds and communicates with the annular chamber 18 and providesthe means through which air is introduced into the annular chamber fromthe source 25. The air introduced is forced past the circumferentiallydisposed registers 21 set at an angle so as to impart a spin to it.Burner means '6 opens into an intermediate annular chamber 27, havingthe frustoconical sides 30. A plurality of angular slots 31 throughwhich the flue gas is introduced are provided in the wall 32 which isdisposed circumferentially about the end of the chamber 27 defined bythe sides 30 and communicate with a second annular conduit 33 which isprovided with regenerator off-gas from the source 34.

To insure maximum conversion of the carbon monoxide to carbon dioxideand thereby the realization of maximum heat value the flue gas should bethoroughly mixed with the hot oxidizing gases produced by the burner.This is accomplished by providing (1) a primary mixing zone produced bythe whirlpool effect given the gases by the angularity of the louvers 21and the angular slots 31 and (2) a secondary mixing zone effected bycreating turbulence in an area 35, of the furnace which is far enoughremoved from the burner tip 6 as to present unstable flame conditions.To assist in the creation of this turbulence an anular ring 36 usuallyat least about 3 inches in height is positioned Within the furnace todeflect gas flow from the sides of thefurnace and thus deflect thecarbon monoxide-containing gas into a zone of turbulence with the streamof hot gases produced by the burner. At the end opposite the burner, thefurnace leads to the conduit 45 which communicates'with heat recoverymeans and provides passage of the hot combustion gases thereto.

Referring to FIGURE 3 the heat recovery means comprises a chamber orconduit 48, preferably provided with insulated walls 50, into which thecombustion gases are introduced and which provides a flow path for thegases. Ordinarily, the cross-sectional area of the flow chamber effectsa maximum gas velocity of about 80 feet per second. This heat exchangeror steam generator is preferably provided with three lower water drums51, 2 upper steam drums 54 and a plurality of tubular conduits 57providing communication therebetween. The upper drums 54 areadvantageously of a larger diameter than the lower drums 51. Two upperdrums with a diameter of about 36 inches and three lower drums with adiameter of about 24 inches are suitable. The drums are cylindrical instructure and extend longitudinally a distance no greater than thatdictated by transportation size specifications, usually a maximum ofabout 30 feet. The walls of the conduit or chamber 48 should be pressuretight for the system operates under positive pressure. Construction ofthe walls can follow conventional boiler practice for pressurizedfurnaces. For example, the walls may be a steel casing with refractorylining to protect them from heat and to prevent heat loss.Alternatively, the walls may be a steel casing protected from the restof the drum by placing boiler tubes against the casing. In this case,the tubes must be tangent or if not tangent must have side studs withthe area between the tubes and behind the studs filled with refractorymaterial. With this design insulation is provided outside the casing tolimit heat loss to the atmosphere.

Water can be supplied to one or both steam drums, as shown in thedrawing, by line 55. FIGURE 1 shows an arrangement by which the fluegases, cooled by passage through the heat exchanger 2 may be conveyedaway by conduit 60 and brought to the economizer 64. This economizer isa heat exchanger which allows downward passage of the flue gas inindirect countercurrent contact to water to be fed to steam generation.Thus, water may enter the economizer from the line 66 and be heated inthe economizer almost to its boiling point before it enters the steamgenerator. The economizer may be of conventional type. Exhaust gaspasses from the economizer to the stack 68 and a conduit (not shown)conducts the steam generated to other parts of the plant where it can beutilized.

The distance which the ring 36 is placed from the CO entry is dependentin part on the height of the ring. When a ring of small height isemployed it is positioned closer to the CO entry; when of greater heightit may be positioned farther from the entry. Generally the annular ringis placed just behind a zone of desired maximum turbulence. This zone isgenerally a paraboloid within the combustion chamber, the surface ofwhich paraboloid may be conveniently defined as the loci around the axisof the furnace of the parabola formed by the intersecting curvesobtained when arcs are drawn from the intersection of the wall 3 of thecombustion chamber with the CO entry wall 32 to the axis of the furnaceusing the opposite intersection as the center of the arc and thediameter of the furnace as the radius of the arc.

Advantageously the minimum placement distance of the ring from the COentry should be approximately at a point where a line extension of thefrusto conical side of annular member 31 intersects the aforementionedarcs. Placement of the ring too close to the burner is undesirable sincethe turbulence created thereby tends to interfere with the flame of theburner. On the other hand the further the ring is placed from the flamethe higher the ring is required to be in order to obtain the desiredturbulence in the minimum space. A large height may be disadvantageousfrom the standpoint of the cost of materials that make up the ring andthe greater tendency of larger rings to erode and crack.

In operation, flue gases from a fluid catalyst regenerator areintroduced by conduit 34 into annulus 33 which imparts a rotary motionto the gases as they enter into the furnace proper via CO entry openings31. The gases are normally at temperatures of about 900 to 1100 F. andissue at high velocities. Auxiliary fuel which can be liquid or gaseousand preferably is natural gas, is fed to the burner means 6 by line 15which is connected to a suitable fuel source. Air to support combustionof auxiliary fuel and the carbon monoxide content of the flue gases isfed to the furnace and burner means through members 12 and 24,respectively. Only a slight excess of air normally is employed to insuresubstantially complete combustion of the carbon monoxide. The airintroduced through inlet 24 is forced past registers 21 which give theair a spin as it enters the furnace. The annular ring 36 deflects theflue gas flowing into the furnace thereby forcing it to mix with the hotgas and create the turbulence desired to combust the carbon monoxide.The combustion gases, after burning, exit from the furnace via conduit45 and enter chamber 43 of the steam generator where they pass in heatexchange relationship with tubes 57 of the steam generator. A watersupply means 58 provides to the steam drums 54, water which can bepreheated by passage through an economizer 64. An economizer is utilizedto cool the exhaust gases in order to increase efiiciency of operation.The outlet water temperature will depend upon (a) the initial watertemperature, (b) the degree of cooling of the exhaust gases, and (c) theinitial temperature of the exhaust gases. The economizer temperature ofthe water is up to about 50 F. of the boiling point. A conduit (notshown) conducts the steam generated from upper drums 54 to other partsof the plant where it can be utilized as desired. Exhausted gasespassing through the steam generation area are passed via conduit 67 intoa stack 68 where they can be released to the atmosphere.

We claim:

1. A heat exchanger comprising three water drums, two steam drumsarranged above said water drums, a plurality of generally parallel heatexchange tubes providing interior communication between water and steamdrums, said heat exchange tubes being enclosed within walls defining achamber for flow of gases normal to the heat exchange tubes, asufficient number of heat exchange tubes being provided to absorb enoughheat from a gas having a heat value of about 15 to 35 B.t.u./ cubic ft.,and containing catalyst fines, to provide about 50,000 to 100,000 poundsper hour of steam at 600 p.s.i.g. and 750 F. while not reducing thecross-sectional area of said flow chamber enough to cause a gas velocityof greater than about feet per second, said heat exchanger being no morethan about 10 feet, 8 inches wide and about 12 feet, 3 inches high.

2. The heat exchanger of claim 1 wherein the three water drums and twosteam drums are outside the walls enclosing the heat exchanger tubes.

References Cited by the Examiner UNITED STATES PATENTS 710,340 9/1902Rust 122352 1,304,998 5/ 1919 Morrison 122352 1,839,125 12/1931 Smith1227 2,421,074 5/ 1947 Kuhner 122347 3,115,120 12/1963 Durham 1227CHARLES J. MYHRE, Primary Examiner.

1. A HEAT EXCHANGER COMPRISING THREE WATER DRUMS, TWO STEAM DRUMSARRANGED ABOVE SAID WATER DRUMS, A PLURALITY OF GENERALLY PARALLEL HEATEXCHANGE TUBES PROVIDING INTERIOR COMMUNICATION BETWEEN WATER AND STEAMDRUMS, SAID HEAT EXCHANGE TUBES BEING ENCLOSED WITHIN WALLS DEFINING ACHAMBER FOR FLOW OF GASES NORMAL TO THE HEAT EXCHANGE TUBES, ASUFFICIENT NUMBER OF HEAT EXCHANGE TUBES BEING PROVIDED TO ABSORB ENOUGHHEAT FROM A GAS HAVING A HEAT VALUE OF ABOUT 15 TO 35 B.T.U./